In a power converter performing PWM control, a structure in which carrier frequency used for PWM control is variably controlled has been known. For instance, Japanese Patent Laying-Open No. 1-321868 (Patent Document 1) discloses a structure for a power converting device performing PWM control, including a series resonance circuit in parallel with a smoothing capacitor, in which carrier frequency for PWM control is variably changed such that resonance frequency of the series resonance circuit and frequency of ripple component included in a DC current of the power converting device inherent to PWM control are matched in a prescribed range.
Japanese Patent Laying-Open No. 6-178550 (Patent Document 2) discloses, as a VVVF inverter current control device, a structure for automatically performing carrier frequency modulation through feedback control such that in accordance with an absolute value of current ripple defined by a deviation between current instruction value of each phase and an actual current value of each phase, the carrier frequency is increased when current ripple is large and decreased when it is small.
Further, Japanese Patent Laying-Open No. 9-37560 (Patent Document 3) discloses an inverter controller capable of reducing current ripple entering the system, in which carrier frequency is variably controlled in accordance with an output of a system voltage detecting means detecting magnitude of system voltage of each phase.
Further, Japanese Patent Laying-Open No. 2006-352951 (Patent Document 4) discloses a control structure for a power converter using first and second inverters, in which phase difference between carrier signals used in respective inverters is controlled, so that ripple current can be reduced as much as possible even when carrier frequency is changed in one of or both of the inverters.
Further, Japanese Patent Laying-Open No. 2005-45848 (Patent Document 5) discloses an AC electric motor controller having a control structure in which, if a difference width between PWM pulses of two phases of a three-phase PWM inverter is equal to or smaller than a prescribed value, or if an output voltage of the three-phase PWM inverter is low, making detection of motor current difficult, the carrier frequency of PWM inverter is automatically reduced to enable current detection.